This invention relates to couplings between a drive shaft and a driven member, and more particularly to a coupling between the drive shaft of a linear motion potentiometer and the mounting for the potentiometer contact spring.
Physical displacements are measured with a linear motion potentiometer by driving a contact spring along resistance and collector tracks housed in the potentiometer by an amount equal to the displacement sought to be measured. The voltage between the contact spring and one end of the element is measured to derive the location of the contact spring, and thereby the amount of displacement. The contact spring is carried by a mounting block, which in turn is moved by a drive shaft extending into the potentiometer parallel to the resistance and collector tracks. The drive shaft is coupled to the mounting block so that, as the shaft moves longitudinally into and out of the potentiometer housing, the contact spring assumes the correct position along the tracks.
In early designs the contact spring mounting was secured directly to the end of the drive shaft inside the potentiometer housing. This arrangement had the disadvantage that the drive shaft had to be precisely aligned with the resistance and collector tracks. A misalignment of the drive shaft from its ideal orientation could result in bending of the shaft, tilting of the contact spring mounting so that it did not ride smoothly through the device, and an uneven contact pressure between the spring and the resistance and collector tracks which increased wear on the spring and tracks at the high pressure areas and tended to make the potentiometer more susceptible to vibrational error at the low pressure areas.
Numerous attempts have been made to solve the problems associated with coupling the drive shaft directly to the contact spring mounting. For example, in U.S. Pat. No. 3,364,454 to Froebe, issued Jan. 16, 1968, the drive shaft extends through the spring mounting. Pivot washers are located between opposite surfaces of the mounting and end flanges or nuts on the shaft. The pivot washers, typically formed from stainless steel with diametrically protruding line contact regions or edges, permit the shaft to pivot away from an orientation parallel to the resistance and collector tracks. A similar approach is taken in U.S. Pat. No. 2,857,497 to M. E. Bourns and E. J. Goeppinger, issued Oct. 21, 1958. In this patent wavy spring washers are interposed between the drive shaft and opposite sides of a wall on the spring mounting, again for the purpose of accommodating misalignments between the drive shaft and spring mounting. In U.S. Pat. No. 2,904,766 to J. C. McMillan, issued Sept. 15, 1959, the drive shaft extends through the mounting and is coupled to the mounting on the near side by a spring loaded washer and on the far side by a ball and socket engagement which again permits limited angular movement of the shaft. Another prior art patent, U.S. Pat. No. 3,250,144 to H. R. Kansman, issued May 10, 1966, discloses a vibration damping coupling intended for a motor vehicle acceleration linkage in which some of the problems faced were similar to those for a linear motion potentiometer. In this patent the drive and driven members are coaxial and separated by an elastomer bushing which is bonded at its outer and inner surfaces to each of the members respectively. The elastomer bushing couples the accelerator pedal to the throttle valve with sufficient resilience in shear to dampen vibrations from the engine and isolate them from the accelerator pedal.
While each of the above devices provides a coupling between a drive shaft and a driven member that permits a limited amount of angular drive shaft movement, they are also subject to some degree of improvement. Metal washers, for example, tend to lose their resiliency after being overstressed and thereafter lose their previous intimate connection between a shaft and spring mounting. Also, it is difficult to adjust to transverse movements of the shaft, since the washers have not been found to move well in a direction transverse to the drive shaft. This latter problem is even more severe with ball and socket type couplings and the elastomer bushing coupling described above. Another serious disadvantage associated in varying degrees with the prior art couplings is that of backlash, i.e., when the drive shaft reverses its direction the spring mounting does not follow the shaft movement precisely. As a result the contact spring may not return to the same location along the tracks when the shaft is moved away from and then brought back to an initial position. This produces an inconsistency between the electrical output signal and the actual position of the shaft.
Of course, it is always desirable that a device be as simple and economical as possible while satisfying its intended functions. In this regard the various prior art devices are all subject to some simplification.